synchrotron radiation is the electromagnetic emission from charged particles gyrating around magnetic field lines — in fusion plasmas, chiefly from electrons — an energy-loss channel that grows with both magnetic field strength and temperature.
The name comes from the machine where the light was first seen: a General Electric synchrotron accelerator, where the visible glow of circulating electrons was observed in 1947. Any charge forced along a curved path radiates. In accelerator rings, electrons at highly relativistic energies emit a spectrum sweeping up to X-rays — the basis of the synchrotron light sources used worldwide for materials and biological science — while in a magnetically confined 📝plasma the gyrating electrons emit at microwave frequencies, at harmonics of the electron cyclotron frequency.
The near-neighbor distinction is against 📝bremsstrahlung: both are radiative losses carried by electrons, but bremsstrahlung comes from deceleration in the electric fields of ions and escapes the plasma freely as X-rays, while synchrotron radiation comes from magnetic gyration and much of it is reabsorbed by the plasma or reflected back by metal walls, so the net loss depends on machine geometry and wall reflectivity. Because the emitted power rises with the square of the field and steeply with 📝electron temperature, synchrotron loss weighs most on high-field, high-temperature designs and on 📝aneutronic fusion concepts operating at billion-degree temperatures. The same emission is put to work diagnostically: electron cyclotron emission radiometry reads a plasma's electron temperature from it.